A radiation therapy apparatus typically radiates the patient from many different angles and with many different amounts of radiation. Before actually treating a patient a treatment plan is determined, which consists of parameter settings for driving the radiation therapy apparatus in such a way that the aimed at treatment effect at a target volume of the patient is achieved. More particularly, the treatment plan is aiming at obtaining the correct dose distribution, and thereby the correct radiation doses at the correct portions of the body of the patient, as prescribed by the physician prescribing the treatment. In other words, a high enough dose is desired in the target volume in combination with as small dose as possible in critical organs. There are many variables which affect the amount of energy that actually is absorbed in the target volume of the patient, and several thereof are not possible to determine accurately, but instead approximating models of them are used. For instance, the determination of the treatment plan can be based on a model of the radiation energy emitted by the radiation therapy apparatus at different settings, a model of how the radiation beams penetrate the body tissue, a description of the patient volume geometry, etc. In order to verify the correctness of the treatment plan, a quality assurance (QA) system is used. The QA system often includes a phantom, which is used for simulating the body of the patient and which is provided with detectors measuring actual doses at positions of the phantom corresponding to the target volume of the body that the treatment is aiming for. If the QA system indicates significant deviations from the desired doses, the treatment plan can be adjusted accordingly.
An example of a known method for determining a patient dose distribution, i.e. the distribution of the radiation dose within the body of the patient, and verifying it, is disclosed in U.S. Pat. No. 7,945,022, where the patient dose distribution is determined by means of phantom dose measurements, and the patient geometry. More particularly, a phantom dose distribution measured by means of detectors at the phantom is compared with a theoretically determined phantom dose distribution, which has been determined by the treatment planning system (TPS). The difference is used for generating a reconstructed patient dose distribution.
The prior art according to U.S. Pat. No. 7,945,022 has the drawback that it is a perturbation method and is hence limited to handle only a small dose deviation correctly. Errors that can be detected in the phantom are classified based on their magnitude into 1) perturbations, i.e. minor disturbances, and 2) “binary” errors, i.e. errors that might or might not be present. Perturbations are exemplified as modelling errors, leaf sequence errors, and the like, whereas binary errors are said to be the selection of incorrect TPS plan file or alignment errors. The idea is that the binary errors will show directly in the measurements and it is hence not meaningful to try to correct the theoretical TPS dose when these errors are present.
While in U.S. Pat. No. 7,945,022 the errors are described as either minor or binary, in the clinical reality there will always be errors that reside in the transition region between two extremes. In this region it is impossible to tell whether the magnitude of the discovered errors in the patient geometry reflects the magnitude of the measured deviations, or if the errors are amplified because the measured dose deviations are too large to fit within the class of minor errors.
Another drawback of the method discloses in U.S. Pat. No. 7,945,022 has to do with its ability to cope with modern rotational therapies, such as arc or helical therapies. The method is based on the construction of error kernels from measured and predicted (TPS) dose in the phantom and the transfer of said dose error kernels from the source of radiation to the patient volume. In order to do this transfer correctly, the position of the source, and hence the angle of beam incidence, that corresponds to a certain dose error kernel, must be known. For rotational therapy, this can only be done when either the TPS dose in the phantom is known on a sub-beam level where the gantry angle is known, or when the spatial distribution of the detectors at the phantom is such as it is possible to determine the gantry angle from which the vast majority of the dose in a detector was delivered.